Electrophotographic printing and copying devices typically are provided with fusing systems that serve to thermally fuse a toner image onto a recording medium, such as a sheet of paper. Such fusing systems normally comprise a heated fuser roller and a heated pressure roller that presses against the fuser roller to form a nip in which the fusing occurs. FIG. 1 illustrates a simplified end view of a typical prior art fusing system 100. As indicated in FIG. 1, the fusing system 100 generally comprises a fuser roller 102, a pressure roller 104, internal heating elements 106, and a temperature sensor 108. The fuser and pressure rollers 102 and 104 comprise hollow tubes 110 and 112 that are coated with outer layers 114 and 116 of elastomeric material.
The internal heating elements 106 typically comprise halogen lamps that uniformly irradiate the inner surfaces of the rollers 102 and 104. Through this irradiation, the inner surfaces are heated and this heat diffuses to the outer surfaces of the fuser and pressure rollers 102 and 104 until they reach a temperature sufficient to melt the toner (e.g., approximately between 160° C. to 190° C.). The fuser roller and the pressure rollers 102 and 104 rotate in opposite directions and are urged together so as to form a nip 118 that compresses the outer layers 114 and 116 of the rollers together. The compression of these layers increases the width of the nip 118, which increases the time that the recording medium resides in the nip. The longer the dwell time in the nip 118, the larger the total energy that the toner and recording medium can absorb to melt the toner. Within the nip 118, the toner is melted and fused to the medium by the pressure exerted on it by the two rollers 102 and 104. After the toner has been fused, the recording medium is typically forwarded to a discharge roller (not shown) that conveys the medium to a discharge tray.
The outer layers 114 and 116 are normally constructed of rubber materials (e.g., silicon rubber) that have high thermal resistance. Where, as indicated in FIG. 1, the rollers 102 and 104 are heated internally, this high thermal resistance creates a heat transport delay that results in a relatively long warm-up time (i.e., the duration of time required for the fusing system to reach operating temperature). The reason for this delay can be explained by the thermal model 200 shown in FIG. 2. The thermal model 200 represents the thermal characteristics of the fuser roller 102 shown in FIG. 1 as a recording medium (e.g., sheet of paper) passes through the nip 118. As indicated in FIG. 2, the model 200 comprises a circuit that includes a thermal energy source 202 representative of the internal heating element 106. The energy source 202 delivers a constant amount of energy to a thermal capacitor C1 that is representative of the hollow tube 110 of the fuser roller 102. The energy provided by the energy source 202 must overcome the thermal resistance provided by the resistor R1, which represents the outer layer 114. Due to the large thermal resistance of the materials used to construct the outer layer 114, the resistance provided by R1 is very large. In addition, the energy from the source 202 must overcome the thermal resistance of the resistor R2, which represents heat loss due to convection. The energy also reaches a second thermal capacitor C2 representative of the thermal capacitance of the outer layer 110. Finally, the energy encounters the thermal resistance of resistor RL, which represents the thermal load of the recording medium that passes through the nip 118. Heat generated by the passage of the energy through the resistor RL is represented by “+” and “−” in FIG. 2.
As will be appreciated by persons having ordinary skill in the art, the large resistance of the resistor R1 poses an impediment to the transfer of energy from the interior of the fuser roller 102 to the fuser roller outer surface of the outer layer. This impediment creates the heat transport delay which is the primary cause of delay in the warming of the fusing system 100. In addition, the high thermal resistance also results in gloss variation along the length of the recording media. As is known in the art, gloss variation relates to the phenomenon in which the gloss of the fused toner changes over the length of the recording medium. This variation is due to the fact that the fuser roller 102 typically has a circumference which is smaller than the length of the recording medium. Therefore, the fuser roller 102 will normally pass through several revolutions as the recording medium passes through the nip 118. Due to the transfer of heat to the medium through each revolution, the temperature of the fuser roller 102 can drop significantly from the leading edge of the medium to its trailing edge. This can result in the printed recording medium having a first section adjacent its leading edge in which the printed media is highly glossy, a second section at its middle where the printed media has a less glossy finish, and a third section adjacent its trailing edge in which the printed media has a non-glossy (i.e., matte) finish.
Gloss variation is undesirable for several reasons. First, printed materials having gloss variation are unaesthetic in that the printed media have an inconsistent appearance. This is particularly true in the case of color printing or photocopying in that the glossy portions of the printed media will appear more vibrant than less glossy portions. Second, a glossy finish normally indicates better fusing to the recording medium. With good fusing, there will be better adhesion between the toner and the recording medium and therefore less chance of the toner flaking off of the recording medium.
A further problem with current fusing systems that incorporate internal heating is temperature overshoot. Temperature overshoot occurs when the temperature of the rollers 102 and 104 exceeds the target temperature set for the rollers. Normally, such overshoot occurs due to the time delay between the application of energy to the rollers 102 and 104 and the temperature response caused by the heat transport delay. Temperature overshoot tends to overheat the toner such that it will not properly adhere to the recording medium. In addition, temperature overshoot causes fusing system degradation in that the temperatures reached by the rollers can cause delamination of the outer layers 114 and 116, thereby significantly reducing the useful life of the fusing system 102.
From the foregoing, it can be appreciated that it would be desirable to have a fusing system that avoids one or more of the disadvantages described above typically associated with internal heating.